The need to obtain information on the performance and remaining lifetime of a tool is of prime importance to many industries. Examples of applications include the manufacturing industry (molds, dies, drilling bits, etc.), the aerospace industry (components of jet engines), the oil industry (drilling equipment), the power industry (vessels and pipes). Such applications call for on-line acquisition of information such as temperature and strain values from tooling and structures. Temperature and strain information can only be obtained by placing sensors into those tools, and information from extended area can only be obtained from arrays of such sensors. Such a solution calls for the placement of the sensors near the points of interest, and therefore the issues of assembly and protection need to be addressed. The assembly of a large number of sensors is cumbersome, time-consuming, and costly, and this endeavor might become difficult for tooling operating in harsh environments. Since the sensors are embedded into functional metallic structure, non-obtrusive embeddability is very important to maintain the integrity of the functional metallic structures. The sensors ought to be small in size and rugged inside the metal matrix. Thin film thermo-mechanical sensors and fiber optic sensors have been identified as two promising candidates.
Fiber optic sensors offer a series of advantages over conventional electronic sensors used to measure temperature, strain, ultrasonic pressure, and other properties. These advantages include small size, high sensitivity, immunity to electromagnetic interference, high temperature capability, multiplexing potential, and low cost.
The small size makes fiber optic sensors good candidates for embedding within structures. Embedded sensors measure parameters at locations not accessible to ordinary sensors, and allow for real-time measurements during fabrication and use of structures. They can also be used for non-contact measurements because they do not require wiring between the sensor and detector. In addition, embedded sensors are protected from damage and isolated from environmental effects to which the structure is subject. While embedding sensors in composite materials is a common process, no successful techniques have been developed for embedding sensors in metal structures with high melting temperatures. During metal casting, in which an enormous temperature change is suddenly applied to the sensor, the sensor undergoes extreme thermal stress and cracks. Silica-based fibers cannot withstand the processing of metals with melting temperatures above 1100° C.
Surviving processing is only one requirement of embedded sensors. Fiber sensors measure strain by measuring fiber displacement, which manifests in a change of a measured property of the light travelling through the fiber. The embedded fiber must, therefore, expand or contract with the metal during measurement, without slipping. There must also be good bonding between the fiber and metal. Without adequate bonding, the fiber slips at the interface during temperature- or stress-induced displacement, resulting in poor measurements.
A method for embedding fiber optic sensors in aluminum, which has a melting temperature of 660° C., has been disclosed by Lee et al., entitled “Method for Embedding Optical Fibers and Optical Fiber Sensors in Metal Parts and Structures” issued in Fiber Optics Smart Structures and Skins IV, SPIE, Vol. 1588, pp. 110-116 (1991). The fiber sensor is positioned in a graphite mold, machined with desired shape, having one optical fiber tube held at one side and a stainless steel held at the opposite side. One end of the fiber sensor is passing through the stainless steel tube and the another end passing through the optical fiber tube. It is believed that the tubes reduce the stress discontinuity at the air-metal interface, allowing the fibers to withstand aluminum casting. Then molten aluminum is poured into the mold. However, Lee's method only works for metals having low melting temperatures. Embedding a fiber optic sensor in a metal structure having high melting temperature will decay the fiber, thus damage the sensor.
Another article entitled “Sensing Applications of Fiber Fabry Perot Interferometers Embedded in Composites and in Metals” by Taylor and Lee, issued in Experiments in Smart Materials and Structures, ASME, AMD-Vol. 181, pp. 47-52 (1993) has disclosed a Fiber Fabry Perot Interferometer (FFBI) as a strong candidate for embedding in a composite or metal part to measure properties of this structure using the method described in the above prior art, “Method for Embedding Optical Fibers and Optical Fiber Sensors in Metal Parts and Structures”. However, the composite layer doesn't bond with nonmetal coating layers of the FFBI, thus the FFBI slips during the measurement.
Furthermore, for metals with higher melting temperatures, for example, stainless steel, nickel, iron, or titanium, all of which have melting temperatures above 1400° C., no solution has been disclosed.
There is a need, therefore, for an embedded fiber optic sensor and a method for embedding a fiber optic sensor in a high melting temperature metal structure, in which the resulting embedded sensor adheres strongly to the metal in which it is embedded.